There are different methods known in the art for determining the flow rate or other different flow parameters. One group of devices to accomplish this task comprise moving mechanical elements such as rotors or the like. Another group of devices comprises instead of the moving mechanical elements at least one pair of transducers acting as a transmitter and a receiver in order to generate, transmit and receive longitudinal waves inside the conduit along at least one transmission path. These devices accomplish different methods in order to determine the different flow parameters.
In a first group of methods the change of transit time is determined which is representative for the flow speed and direction. As disclosed in U.S. Pat. No. 4,015,470, selected variables of a fluid flowing through a conduit are measured by transmitting acoustic waves through the conduit wall and fluid stream along normal and oblique transmission paths between acoustic transducers located externally of the conduit, measuring the transit times of the waves between the transducers, and combining these transit times with certain conduit and transducer parameters according to predetermined mathematical relationships to obtain the values of the variables. The variables which may be measured are flow velocity, mass flow rate, sonic velocity of the fluid, fluid compressibility, fluid temperature, and fluid density. CH 669 463 suggests a method for measuring the flow rate, the volume flow rate, the temperature and the average molecular mass of gases or gas mixtures in a pipe on the basis of measuring the transit time with two transducers. The measured value of the transit time is dependent to a large extent on the speed of the acoustic waves and therefore on the medium properties such as temperature, pressure and others.
In a second group of methods the change of frequency is determined which is representative for the flow speed and direction. U.S. Pat. No. 3,987,673 discloses a flowmeter for measuring blood flow in a vascular channel by Doppler Techniques has an assembly of electro-ultrasonic transducers which direct ultrasonic radiation to a zone spaced from the assembly and receive radiation reflected therefrom. The transducers are spaced apart and capable of providing two radiation beam paths from transmitting to receiving transducers after reflection from the zone. The transducers are directed such that part of one of the beam paths forms an acute angle with the direction of blood flow and part of the other beam path forms an obtuse angle with the direction of blood flow. The reflected signals are shifted in frequency by Doppler effect and their instantaneous difference frequency is monitored and expressed directly in terms of velocity. Measuring only one frequency change has the same drawback as measuring a change of one transit time, it results in a dependency on sound velocity. However when measuring two frequency changes and determining the difference, this kind of measuring error can be eliminated. A further problem arises however when continuous waves are used, which cause unintended interferences of reflected waves and the forming of standing waves which deteriorate the overall result of measurement.
In a third group of methods the change of the phase of the transmitted wave is determined which corresponds to a change of transit time being representative for the flow speed and direction. An acoustic flow meter is known from U.S. Pat. No. 4,616,510 in which a plurality of different frequencies are transmitted in both upstream and downstream directions, and corresponding differences in transmission phase-delays due to such frequency changes detected and used to compute the fluid velocity by the formula vF=πL (1/MD−1/Mu) when L is the distance between transmitter and receiver, MD is the slope of the graph of phase delay vs. transmitted frequency for the upstream direction of transmission, and Mu is the corresponding slope for the downstream direction. A signal processor using a least-squares algorithm is preferably employed to obtain best values for MD and Mu from the several values of phase-delay difference produced by the several different transmitted frequencies. When using different frequencies, it might be difficult to avoid unintended interferences of the different transmitted and reflected waves which could result in non-systematic measuring errors.
U.S. Pat. No. 4,633,719 discloses a method and apparatus for generating a signal proportional to flow in a pipe using probes strapped onto the exterior of the pipe. Two ultrasonic wave packages at a carrier frequency are sent in opposite directions from one probe to the other. Two lower frequency signals are extracted from the two respective wave packages and analyzed to detect a phase difference. The phase difference corresponds to a difference in the travel times for the two wave packages caused by the flow velocity in the pipe. The two lower frequency signals are converted to digital data samples which are stored with reference timing data. A microelectronic processor synchronizes the two sampled waves, using the timing data, before comparing the sampled waves to determine the phase difference. The processor and associated circuitry also measure the travel times of the two ultrasonic wave packages and generate an output signal proportional to flow based on the measured phase difference and the measured travel times. The suggested arrangement comprises only one pair of transducers which are used as a transmitter and a receiver alternately. This arrangement does not allow a continuous measurement and a relatively large time interval has to be selected between two consequent measuring cycles due to the recovery time of the transducers. Due to this fact, this method and apparatus always will be imposed with a substantial systematic error.
Therefore it is an object of the invention to provide a method and apparatus for determining the flow parameters of a streaming medium in a conduit with a minimal or no dependency on the velocity of the acoustic waves in the medium and on the different medium parameters, such as temperature, pressure and others. A further object of the invention to provide a method and apparatus for determining the flow parameters of a streaming medium in a conduit with a high accuracy and resolution throughout the whole measuring process and no or a minimum of systematic and non systematic errors. It is also an object of the present invention to provide a method and apparatus for determining the flow parameters of gases, especially of air in a wide range of flow rate and with different streaming directions and other boundary conditions, such as gas composition, humidity, temperature, ect. In case of medical applications the conduit should be replaceable and sterilizable.